Shim coils

A set of shim coils usually represents the field expansion according to spherical harmonics. Depending on the required accuracy, more or fewer orders of the expansion are used. Clinical imaging systems usually use only the first order... 

Cryoshims Rough (superconducting) shim coils that are built into superconducting magnets and adjusted at installation of the instrument. 

Shim coils Coils built into NMR magnets designed to improve the homogeneity of the magnetic field experienced by the sample. Two types of shims are used cryoshims and room temperature shims. Normal shimming involves the use of the room temperature shims. 

Shimming The process of adjusting current flowing through the room temperature shim coils in order to achieve optimal magnetic field homogeneity prior to the acquisition of NMR data. The process may be performed manually or under automation. 

The magnetic field inside a superconducting solenoid is not particularly homogeneous, and compensation of the field gradients is quite a complex problem that has been solved49 by the provision of mathematically designed arrays of superconducting or normal, resistive shim-coils, or both. 

Second, the contour (shape) of the field itself can be varied (within very narrow limits) by passing extremely small currents through a complex series of shim coils located around... 

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